System and Method for Providing Uniform Brightness in Seam Portions of Tiled Images

ABSTRACT

The present invention provides method for producing a substantially seamless video image on a display surface. The method comprises the steps of separately projecting at least a first and a second video image onto a display surface such that a seam is defined by overlapping portions of said first and second video images. Inside the seam, the brightness of video image is adjusted by adjusting pixels of said overlapping portions in accordance with a modulus determined by the number of overlapping portions defining the seam.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to provisional application Ser. No. 60/751,336 filed Dec. 16, 2005, incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to projection displays, and more particularly to projection display systems comprising a plurality of projectors arranged to display tiled images on a display screen.

BACKGROUND OF THE INVENTION

The quality of a projected image can be described by reference to a number of image characteristics. Each characteristic represents a potential source of distortion in the displayed image. Brightness and brightness uniformity are important characteristics of displayed images. Brightness distortion, also referred to as luminance distortion degrades the quality of a projected image to a viewer of the image. Most projectors do not project images at a constant luminance level across the entire display screen. Therefore, brightness distortion is a common problem in the design of projector display systems.

Brightness distortion has many possible sources. A common source of brightness distortion is due to inherent optical characteristics of lenses used in projection displays. This non uniformity is due to the design of the optics within the light engines of the projectors. Another possible cause for non uniformity is the projection lamps themselves. Regardless of the source of the luminance distortion, non-uniformity in luminance detracts from the displayed image in the eyes of a viewer of the image.

The luminance non-uniformity of a projector and its associated lens can become more pronounced when a plurality of projectors are employed in combination to display a single image on a display screen. Such an arrangement of projectors is as “tiling”. Tiling projectors and projector images on a display screen provides a larger image with higher overall resolution than can be obtained from a single projector. However, the technique of tiling images for display has drawbacks. Non-uniformity in luminance is often much more apparent in a composite image created by multiple projectors whose individual images are tiled together. This is particularly a problem in “seam” areas of the displayed image. Seams are created in those areas where images from a plurality of projectors overlap each other on the display screen. Brightness non uniformity in seam areas of a displayed tiled image is distracting to viewers and degrades the quality of the displayed image. Therefore, systems and methods for maintaining brightness uniformity in seam areas of tiled images are needed.

SUMMARY OF THE INVENTION

The present invention provides systems and methods for maintaining brightness uniformity in seam areas of tiled images.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be described below in more detail, with reference to the accompanying drawings, in which:

FIG. 1 is a block diagram of an example display system including two projectors according to an embodiment of the invention.

FIG. 2 is a block diagram of an example display system including four projectors according to an embodiment of the invention

FIG. 3 is a flow chart illustrating steps of a method for smoothing seams in a tiled display according to an embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 illustrates a display system 10 for projecting a video image 20 onto a display screen 40. System 10 comprises a plurality of video projectors (160, 260) arranged such that each projector (160, 260) separately projects first and second portions (158, 258 respectively) of image 20 onto display screen 40. In the embodiment illustrated in FIG. 1, projectors 160 and 260 comprise spatial light modulator (SLM) type projectors. According to one example embodiment of the invention projectors 160 and 260 comprise DLP™ projectors. DLP is a trademark of Texas Instruments. The light engine for each projector comprises any suitable technology, such as one or more Liquid Crystal Display (LCD) panels, Digital Light Processing (DLP) or Liquid Crystal on Silicon (LCOS). Nonetheless, those skilled in the art will recognize-that the present invention may be used. with other projectors, including those using other types of image generation technologies.

Each separately projected image portion 158, 258 comprises a corresponding unique image portion (159, 259) respectively of image 20. Each separately projected image portion 158, 258 further comprises a common image portion (60L and 60R) respectively of image 20. For example, a first projector 160 projects first portion 158 of image 20 onto screen 40. First portion 158 of image 20 comprises a unique image portion 159, i.e., a portion of image 20 that is not projected by any other projector. First portion 158 of image 20 also comprises a common portion 60L. Portion 60L is a duplicate of image portion 60R of image portion 258. Common portion 60L and common portion 60R are projected to overlap each other on display 40. The overlapping common portions 60L and 60R define a seam 88 of projected image 20.

System 10 further comprises a video image separator 90. A video signal representing video image 20 is provided to video image separator 90. A processor 95 of image separator 90 separates the incoming video image signal into video image signal portions 106 and 206. Video image signal portions 106, 206 represent video image portions 158 and 258 respectively.

Video image separator further comprises a pixel brightness adjuster 27. Pixel brightness adjuster 27 examines pixel values within overlapping image portions (e.g., 60L and 60R) of image signal portions 106 and 206. Brightness adjuster 27 divides pixel values corresponding to overlapping pixels of overlapping image portions 60L and 60R (for example pixel 32 and 22) by the number of overlapping image portions comprising seam 88. In the system illustrated in FIG. 1 seam 88 comprises two image overlapping image portions 60L and 60R. Therefore pixel brightness adjuster divides the brightness value for pixel 60L and 60R by two. When projected onto display 40 the combined brightness of pixels 60L and 60R will approximate the intended brightness corresponding to pixel 60 of input image 20.

However, pixel values are represented by a limited number of bits. For example pixel values are commonly represented by 8 bits. Each of the 256 combinations of 8 bits corresponds to a different brightness level. If a brightness level does is not evenly divisible by the number of projectors it is not possible to accurately represent the original brightness value by a combination of equal lower values. In order to more closely approximate the original brightness value, pixel adjuster 27 determines the modulus (n) of the pixel brightness value to be adjusted, where n is the number of projectors comprising system 100. Pixel brightness adjuster 27 adjusts the pixel brightness value for each projector based on the modulus (n) it determines.

In a two projector system pixel brightness adjuster 27 determines the modulus (2) of pixel brightness values of overlapping pixels. If the modulus (2) is 0, the original brightness value is evenly divisible by the number of projectors. In that case dividing the value by two and assigning equal values to each overlapping pixel will provide the original brightness value when the overlapping pixels are displayed.

If the modulus is 1, one of the overlapping pixels is assigned the integer portion of the original brightness value divided by the number of projectors. The other overlapping pixel is assigned a brightness value equal to the brightness value assigned to the other plus 1.

FIG. 2 illustrates a projector system 100 according to an alternative embodiment of the invention. Projector system 100 comprising four projectors 160, 260, 360 and 460. Incoming video signal 12 is provided to video image separator 90. Video signal 12 represents an image 20 to be displayed on screen 40. Image separator 90 comprises a processor 95 and a pixel brightness adjuster 27. Image separator 90 separates incoming video signal into video signal portions 81, 82, 83 and 84. Each video signal portion represents a portion of image 20. Similar to the embodiment of FIG. 1 each image portion 159, 259, 359 and 459 of image 20 comprises a unique image portion and an overlapping image portion. Overlapping image portions define seams 55, 66, 77 and 88 of displayed image 20. In a configuration comprising four projectors arranged in accordance with FIG. 2, an image area 73 comprises four overlapping image portions. Accordingly the brightness of a given pixel, for example pixel 800, in image area 73 will be a combination of four pixel brightness values, one value supplied by each video signal portion 81, 82, 83 and 84.

Pixel adjuster 27 of video separator 90 compensates for distortions in brightness by dividing the brightness value P of each pixel in area 72, for example the brightness value of pixel 800, by four (the number of projectors providing a pixel value for pixel 800). A value of P/4 is assigned to each overlapping pixel. To avoid loss of dynamic range pixel adjuster 27 also determines the modulus (n) for pixel brightness values of overlapping pixels, where n=4. If the modulus (4) of the brightness value is 0 each of the four overlapping pixel values is assigned a brightness value equal to the original brightness value divided by 4 (P/4). If modulus (4) of the brightness value is 1 a binary 1 is added to P/4 for one of the four overlapping pixels. If the modulus (4) of the brightness value is 2 a binary 1 is added to P/4 for two of the overlapping pixels. If the modulus 4 of the brightness value is 3 a binary 1 is added to three of the overlapping pixels. By adjusting pixel brightness values in accordance with the modulus (n) of the brightness of overlapping pixels, pixel brightness adjuster improves the dynamic range in the seams of image 20. 

1. A projector system for displaying images on a display screen the system comprising: An image separator coupled to receive an input video signal representing an image to be displayed and to provide separate output video signals corresponding to portions of said image to projected on said screen; Said image separator adjusting pixel values of overlapping image portions in accordance with a modulus (n) of said values where n=the number of projectors comprising said system.
 2. A method for improving dynamic range in seams of tiled image projected onto a display screen comprising steps of: separating an input image into at least two image portions, each of said image portions including at least one image area comprising pixels corresponding to pixels of the other image portion; for each of said corresponding pixels dividing pixel brightness value by the number of corresponding pixels to obtain a divided brightness value; for each of said corresponding pixel brightness values determining a modulus n of said value wherein n=the number image portions; adjusting brightness values of said corresponding pixels based upon said modulus. 